The invention pertains to a cutting tap that is used to cut internal screw threads in a material. More specifically, the invention pertains to a cutting tap that is used to cut internal screw threads in a material wherein the cutting tap exhibits a cutting edge geometry that makes it suitable to be made from cemented carbide (e.g., cobalt cemented tungsten carbide) tool materials.
Mechanisms and machine components requiring screw threads have had a long history in many technologies. In this regard, the use of screw threads as a fastener component has consistently dominated over all other means to join component parts into assemblies. Thus, it can be appreciated that articles used to form screw threads are essential to many manufacturing technologies.
Although there are many ways to generate internal screw threads, as well as external screw threads, experience has shown that cutting taps are the favored way to generate an internal screw thread. There currently exist two tapping methods to generate internal screw threads. The dominant one of the tapping methods comprises cutting material from the wall of a hole and then removing the same to produce a helical V-shaped screw thread. In the alternative, material can be displaced to form an internal screw thread. The first method of tapping by cutting material is generally favored over the second method since the method of tapping by cutting requires lower torque and produces a more precise thread form than does the displacement method.
As can be appreciated, the dimensional accuracy of the shape and size of the internal screw thread controls the precision and fit of the screw thread assembly. In other words, the ability to form an internal screw thread that exhibits improved dimensional accuracy with respect to the shape and size of the threads results in a more secure connection between the components of the assembly. It thus becomes apparent that it would be highly desirable to provide a cutting tap, as well as a method of cutting internal screw threads, that produces an internal screw thread with improved dimensional accuracy with respect to the shape and size of the threads.
As can also be appreciated, the speed of tapping affects the cost to produce an internal screw thread. What this means is that a manufacturing advantage can exist due to the use of a cutting tap that can produce internal screw threads at a higher rate or speed than has previously existed. It thus becomes apparent that it would be highly desirable to provide a cutting tap, as well as a method of cutting internal screw threads, that produces an internal screw thread at a higher speed than has heretofore been available.
The dominant material used to manufacture cutting taps is high-speed steel. High-speed steel exhibits good strength properties, which make it suitable for manufacture as a cutting tap in light of the geometric design thereof. Exemplary high-speed steels suitable for cutting taps include grades such as M1, M2 and M7. See ASTM Specification for Tool Steel High Speed.
In the context of other kinds of cutting tools (e.g., cutting tools used for chip forming material removal applications), cemented (cobalt) tungsten carbide is a preferred material for manufacturing such cutting tools over high-speed steel due to properties such as, for example, higher hardness and better high temperature stability including the ability to retain its hardness at high temperatures. Typically, cutting tools manufactured from cemented carbides (e.g., cobalt cemented tungsten carbide) can be used at cutting speeds that are at least three times higher than cutting tools manufactured from “high-speed” steel. In addition, the useful tool life of the cemented carbide cutting tool has been longer than the useful tool life of the high-speed steel cutting tool. It can thus be seen that in chip forming material removal applications, the use of cemented carbides provides meaningful advantages over using high-speed steels.
While cemented carbide has provided these advantages in the context of a chip forming material removal application, heretofore, cemented carbide has not been a preferred material from which to form a cutting tap. Since cemented carbide has a lower strength than high-speed steel, its use for cutting taps is limited because cutting taps are typically geometrically weaker than other cutting tools.
In light of the advantages that exist with using cemented carbide cutting tools in other cutting applications (e.g., chip forming material removal applications), it would be desirable to provide a cutting tap that can be made from cemented carbide. Such a cemented cutting tap would provide numerous advantages over existing high-speed steel cutting taps. In this regard, a cemented carbide cutting tap would result in an improvement of the dimensional accuracy with respect to the size and shape of the threads as compared to high speed steel cutting taps. A cemented carbide cutting tap would result in an increase in the useful tool life of the cutting tap as compared to high speed steel cutting taps. A cemented carbide cutting tap would increase the production speed for internal screw threads as compared to a high speed steel cutting tap.